Ecological research uses eBird data to show that migratory birds are falling out of sync with the timing of spring

Northern Parula
Setophaga americana

At eBird, our goal is to connect valuable birdwatcher sightings with research and conservation. The eBird checklists that you’ve entered have been used in over 100 peer-reviewed papers, and hundreds of local, regional, and national conservation decisions. We’re excited to feature one of the most recent papers published on eBird, “Increasing phenological asynchrony between spring green-up and arrival of migratory birds”. Read on to see lead author Stephen Mayor’s story of how eBird data helped illuminate an increasing mismatch between when plants green up and when migrant birds return in spring.

Spring has sprung in many parts of North America, and a great joy is hearing the neotropical migrants return to fill the forests, fields, and backyards with song. The sequence seems timeless: the weather warms, the trees leaf out, and then the birds arrive. But the timing of spring changes from year to year and so I wondered how well birds were able to track this change, especially given the forecasts for a warmer and more variable climate.

With an international team of researchers, I led a study using eBird observations to examine how well birds are keeping up with the timing of Spring green-up. eBirders are familiar with BirdCast migration forecasts and know that migration timing is linked to the weather from year to year. Of course, it’s not just the weather that birds are concerned about: birds are hungry after their long migratory flights! The timing of green-up is critical for birds because when the trees leaf out, caterpillars emerge to feed on the young leaves, an important source of protein that contributes to survival and reproduction. Arriving at the optimal time is vital: birds must arrive late enough to avoid frigid conditions but early enough in time to catch the spring pulse in food and establish nest sites and territories.

On average, the 48 songbird species we investigated were falling out of sync with the timing of spring green-up by 5 days per decade. Some species were becoming increasingly mismatched to their environments by double or triple that rate.

We found markedly different patterns in eastern versus western forests. In the eastern temperate forests from southern Canada to Florida, spring green-up generally advanced. Birds like Northern Parula and Yellow-billed Cuckoo also typically arrived earlier, but did not keep pace with the change of spring. In the western forests, spring green-up unexpectedly became later over the period between 2001-2012, and birds like the Townsend’s Warbler also arrived later. Just as in the east, they adjusted their arrival times in the right direction, but didn’t keep pace with changing green-up.

The increasing phenological mismatch may contribute to bird population declines, something the researchers will be further investigating. It’s also not clear why some species seem to be growing out of sync with their environments while others seem to be doing just fine.

How did we estimate the arrival of bird populations? We don’t just take the first sighting of a species to any given area, which could be inaccurate if birders aren’t watching everyday in every place. Instead we look at how observations ramp up over the spring to get an average arrival date of the population.

We greatly appreciate the time and care everyone takes to submit accurate observations. Together, we’re contributing to a better understanding of avian ecology that could ultimately benefit bird populations.